We also need to wrap realloc() to dispatch pointers to the correct
free/alloc implementation.

malloc/calloc/realloc/posix_memalign are simply replaced by
their external counterparts.

free() gets a wrapper which route the pointer to the hopefully
correct free() implementation.

This lets library users override them via the usual compiler include
path.

Signed-off-by: Simon McVittie <smcv@collabora.com>

While there, drop unused $(shim_files) and $(shim_srcs) variables.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Signed-off-by: Simon McVittie <smcv@collabora.com>

The generated shim libraries (libz and libGL are included as examples)
provide the same symbols as the target library but do not expose the
symbols from their dependencies to other objects that link to them.

They are also capable of using a filesystem tree starting somwhere
other than / to get the dependencies for the target.

Included are tools to help generate said proxy libraries with minimal
manual intervention.